This invention relates to the manufacture of steels that have a high nitrogen content. More particularly, the invention is directed to a method of making ultra-low carbon enameling steels that are stabilized for good formability and that have a high nitrogen content for excellent enameling characteristics.
For many uses, enameling steel must be of a high grade with sufficient formability and drawability to be molded into, for example, bath tubes, sinks and the like. To impart suitable formability characteristics to the steel, the steel is stabilized with reactive alloying elements such as titanium, columbium and boron. In the past, stabilized enameling steels have contained on the order of 0.02% carbon. The level of reactive alloying elements necessary to stabilize this level of carbon resulted in significant quantities of deoxidation products, such as alumina, contained in the immediate subsurface of the steel. In order to make a satisfactory product for many of the applications to which such steels were to be applied, it was necessary to completely remove this surface at significant cost in manpower and yield. The problems associated with surface defects in the stabilized steel can be reduced or eliminated by utilizing stabilized ultra-low carbon (ULC) steel i.e., steel containing only about 0.005% carbon. Steels containing only about 0.005% carbon can be stabilized with smaller amounts of stabilizing elements and thereby provide the desired formability and drawability properties without the associated surface defects. However, while ultra-low carbon chemistry provides the necessary formability and surface characteristics, ULC steel alone will not suffice for enameling purposes. Steel that is to be enameled must generally have the ability to resist the formation of so called "hydrogen defects."
The presence of moisture during the enameling of the steel will inevitably result in a certain amount of hydrogen being dissolved in the solid steel. Unless the steel contains a component or components that will scavenge and hold the hydrogen within the steel, the hydrogen will gradually escape from the steel and cause defects in the overlying enamel that is subsequently coated thereon. The most problematic hydrogen defect occurring in the enamel is known as "fish scale." Since this problem may not appear for days or weeks after the steel is enameled, the defective steel may already be incorporated into the final product and installed in, for example, a new home before the it manifests itself. This results in significant losses in terms of time, manpower, productivity and ultimately cost to the steel manufacturer, the product manufacturer and the consumer. Another enameling defect manifests itself as bubbles or discolorations in the overlying enamel.
In order to obtain satisfactory enameling properties in stabilized ultra-low carbon steel it has been found that a high nitrogen content is extremely useful. While it is normally desirable to maintain a low nitrogen content in ULC steel, a sufficiently high nitrogen content has been found to reduce or eliminate hydrogen defects by forming hydrogen scavenging reaction products such as TiN, ZrN and BN in the steel. These reaction products prevent the hydrogen from escaping and causing defects in the overlying enamel.
One way to get nitrogen into the steel is by adding nitrogen containing alloys, such as nitrided manganese and nitrided calcium after the oxygen blowing cycle in a Basic Oxygen Furnace (BOF). However, since these alloys are quite expensive they increase the cost of the process and the steel. Such alloys also tend to distort the carbon/oxygen ratios in the steel so that there is frequently insufficient oxygen present to process the steel to ultra-low carbon levels by vacuum circulation decarburization. Nitrogen can also be added in the vacuum degassing process by using nitrogen instead of argon for the lift gas from the tuyeres in the so called "up leg" snorkel of an RH degasser. However, the recovery is variable and will not provide an adequate nitrogen content to prevent hydrogen defects. Nitrogen can also be added in the BOF using the inert gas tuyeres. However, the results will again be variable and insufficient to achieve the target chemistry. Although combinations of these practices may, on occasion, result in adequate nitrogen and carbon in the product, the results of the combinations, as with the individual practices, will be variable and insufficient to make the necessary steel chemistry with adequate reproducibility.
In order to adequately address the formability requirements of high end enameling customers, while at the same time provide a steel which contains sufficient hydrogen absorption capability to avoid fish-scale and other enameling defects, it is desirable to use fully stabilized ultra low carbon content steel to achieve suitable formability and, at the same time, have nitrogen values in excess of 0.01% to form the inclusions necessary to hold excess hydrogen. This is a significantly higher nitrogen content than normal ultra-low carbon steel, which is typically only on the order of 0.006% and below. This combination of requirements is unique to enameling steel and necessitated the development of the inventive process. Prior to the inventive method, nitrogen contents could not be maintained at a high enough level to make ultra low carbon enameling steel.